A storage system for one or more hosts typically comprises a number of ports via which data write or read requests are transferred to or from the hosts. The storage system consists of one or more logical volumes or units, and the data requests from the hosts are directed to these logical units via the ports. The logical units comprise non-volatile media wherein data of the storage system is stored, and since these non-volatile media typically have slow access times, one or more fast access time caches are normally interposed between the ports and the non-volatile media. The caches reduce bottlenecks that would be caused by the slow access time of the non-volatile media. Typically, each logical unit has its own command queue, wherein the data write and read requests, as well as other commands, are placed. Different requests and commands to a specific logical unit may arrive via different ports. However, the storage system needs to preserve the order of these different requests and commands to avoid inconsistencies accruing in the storage system. To preserve this order, the storage system typically uses a shared resource, such as one or more central shared memories, which monitors requests and commands arriving at the ports, and creates the command queues of the individual logical units.
U.S. Patent Application 20010028524 to Hoskins, whose disclosure is incorporated herein by reference, describes a method for selecting optimal command nodes (a data structure consisting of a number of fields which contain information necessary to carry out a command in a microprocessor). The method is implemented for a computing device by identifying a command node in a first queue and determining if the identified command node collides with a command node in a second queue. If a collision between the identified command node and a command node in the second queue is determined, the collision is corrected and the identified command node may then be moved into the second queue. The second queue is then sorted according to a predetermined routine to select the optimal command node.
As storage systems increase in size, the number of requests and commands also increases. Hence the number of ports which shared resources of the system have to monitor typically increases and the operation of the shared resources becomes more complex. The increasing complexity creates its own bottleneck, so that full scalability of ports is not achievable. A system which avoids the complexity of shared resources, and which is also completely scalable with regard to ports, would therefore be advantageous.